Well first off... louder is not better and any mastering engineer will tell you - you loose quality with volume. The nuances and details of your song is lost.

Unfortunately today's trend is to get as loud as possible which requires a compressor. More compression flattens the song to remove the peaks and consequently removes the dynamics of a song.

Clipping is only your concern when recording and that depends on your mic and equipment. You'll know it when you hear it. In mastering you're worried about distortion; that point at which the audio is being destroyed. We'll call it clipping for now to help explain:

The clipping can be all but eliminated even above 0db with proper compression. 0db was the limit that analog tape could withstand cleanly. Today 0db is the limit at which digital audio distorts. Most songs can peak at 2 or 4 without clipping. Most studios work at -4 db; mix the song and the Mastering Engineer then compresses the song into a flat shadow of itself to reduce the peaks and bring the whole song up to above 0. Mastering Engineers do not like to do this. They are audio geniuses with well trained ears being forced to reduce audio quality.

Only you can tell for sure what your limit is - depending on recording quality, mixing and etc. Play your music back in several different devices including your monitors, your home stereo, your car and your ipod. Also compare it to some reference material like your favorite CD.

Just remember that you loose a lot of your song the louder you go. Be sure to post your music here for us to listen to.
Justin